Many applications use converters such as Digital-to-Analog Converters (DACs). High-speeds and high resolutions are needed for DACs used in applications such as for industrial, medical, automotive, and consumer areas.
Current-cell DACs have many current cells that turn currents on or off by a digital code. The currents are summed to create a combined analog current that is the analog representation of the digital code. The combined analog current may be converted to an analog voltage by passing the current through a resistor.
A thermometer-code DAC has a total of 2N−1 current cells that each produce a same current. A binary-weighted DAC has current cells with currents that double for each successive current cell. A hybrid DAC has some binary-weighted current cells, such as for most-significant-bits (MSBs), and some equal-current cells, such as for least-significant-bits (LSBs).
FIG. 1 shows a current-cell DAC. Current sources 31, 32, 33, . . . 35, 36 all have equal currents of I0. A digital input is converted to a thermometer code S1, S2, S3, . . . S(2N−2), S(2N−1) to control switches 21, 22, 23, . . . 25, 26. Each switch causes the entire cell current I0 to switch either to true output OUT+ or to complement output OUT−. Resistors 10, 12 between an analog power supply voltage AVDD and OUT−, OUT+ produce analog voltages that are converted from the digital value represented by the thermometer code.
FIG. 2 is a graph of the spectral performance of a DAC. The signal strength of the output of the DAC is highest at a fundamental frequency F0. The signal strength is lower for other frequencies of operation, but is higher at spur frequency F1. The difference in signal strength between fundamental 14 and spur 16 is the Spur-Free Dynamic Range (SFDR). A larger SFDR is important for better high-speed performance of a DAC.
Larger spurs often occur at harmonics of the fundamental frequency, especially the second and third order harmonics. A higher output impedance of the current cells can improve the SFDR.
The inventors have realized that the output impedance varies with the digital code being converted. The code-dependent output impedance is a cause of harmonic distortion that increases the signal strength of spurs and thus decreases the SFDR and reduces high-speed performance of DACs.
What is desired is a DAC that has an improved high-speed performance. A DAC with a high output impedance that does not vary with the digital code is desired. A DAC with reduced spurs at harmonics is desirable. A DAC is desired to be constructed from current cells that have a constant output impedance, regardless of the digital code being converted.